This invention relates to an improvement in the long used method of making block copolymers from conjugated diolefins and/or alkenyl aromatic hydrocarbons. The improvement comprises utilizing hydrogen gas to terminate the anionic polymerization.
Anionic polymerization utilizing organo alkali metal initiators has long been used to polymerize conjugated diolefins such as butadiene and isoprene and to copolymerize such diolefins with styrene and substituted styrenes to make styrene-butadiene-styrene (S-B-S) and styrene-isoprene-styrene (S-I-S) block copolymers and similar block copolymers. The reaction of these latter block copolymers is used below for exemplary purposes. This reaction is usually carried out in an inert hydrocarbon solvent such as cyclohexane or toluene and it is necessary to rigorously exclude oxygen, water or any impurity that can react with the highly reactive propagating species. Under these conditions the polymeric molecular weights can be precisely controlled. The preferred initiators are organolithiums, although others can be used. Two commonly used methods are:
1. Sequential. i.e., start polymerization at one end of the molecule and continue to the other.
2. Coupling. i.e., start polymerization at each end of the molecule and then join the reactive chains together by a coupling or a linking agent.
In these polymerization methods, sec-butyl lithium is the preferred initiator because it initiates the polymerization very readily. That is to say, the rate of the initiation reaction is high compared to that of the subsequent polymerization. This initiator first reacts with one molecule of styrene monomer. This is known as the initiation reaction. ##STR1##
The product can then continue polymerization of the styrene and this is known as the propagation reaction. ##STR2##
The new end product is termed polystyryl lithium (the effects of the terminal sec-butyl radical are ignored) and it is denoted as S.sup.- Li.sup.+. If a diene (in this case butadiene) is added, the S.sup.- Li.sup.+ can initiate further polymerization: EQU S.sup.- Li.sup.+ +n(CH.sub.2 .dbd.CHCH.dbd.CH.sub.2).fwdarw.S(CH.sub.2 CH.dbd.CHCH.sub.2).sub.n-1 CH.sub.2 CH.dbd.CHCH.sub.2.sup.- Li.sup.+( 3)
For the above reaction the product is denoted S--B.sup.- Li.sup.+. It also is an initiator, so that if more styrene monomer is now added, it will polymerize onto the "living" end of the polymer chain: ##STR3##
When this last reaction is complete, the product (S--B--S.sup.- Li.sup.+ -polystyryl lithium) can be inactivated by the addition of a protonating species such as an alcohol. This terminates the reaction: EQU S--B--S.sup.- Li.sup.+ +ROH.fwdarw.S--B--SH+ROLi (5)
If the polymer is to be made by coupling, the first three reactions shown above are unchanged, but instead of the S--B.sup.- Li.sup.+ initiating further polymerization of styrene, in this case it is reacted with a coupling agent: EQU 2S--B.sup.- Li.sup.+ +X--R--X.fwdarw.S--B--R--B--S+2LiX (6)
Many coupling agents have been described, including esters, organohalogens and silicon halides. The example above shows the reaction of difunctional coupling agents but those of higher functionality (for example SiCl.sub.4) can also be used and give branched or star-shaped molecules (S--B).sub.n x. There are cases whereby the coupling agent is not incorporated in the polymer. If divinyl benzene is added at the end of the reaction the products are highly branched, i.e., the value of n is very large. This reaction can also be terminated with an alcohol. It is necessary to terminate the living polymer to prevent crosslinking and unwanted coupling reactions, and hence formation of high molecular weight polymer.
The use of alcohol results in formation of alkali metal alkoxides and excess alcohol impurities. The excess alcohol and alkali metal alkoxides adversely affect the activity of some hydrogenation catalysts in the downstream hydrogenation step should hydrogenation of the polymer be desired. Additionally, residual alcohol in the polymerization reactor deactivates part of the living polymer in the next batch which can lead to poor molecular weight control through the formation of intermediate molecular weight material and/or polystyrene homopolymer. Also, in using methanol as a polymerization termination agent it is required that the majority of methanol be removed from recycled solvents creating waste effluent which must be disposed of. Thus, there is a need for a method of terminating the polymerization of these living polymers which would not result in the formation of alkali metal alkoxides and excess alcohol in the system. The termination step of the present invention is clean and efficient, and produces an impurity free polymer cement.